In a semiconductor device, an insulation zone called a depletion layer is formed at a p-n junction. When a width of the depletion layer is small, electric field strength in the depletion layer becomes high. When the electric field strength is too high, insulation breakdown occurs, which leads to damages in an element. It is known that the width of the depletion layer becomes narrow at an edge of an active region, and electric field strength becomes high therein.
As a technique to enhance dielectric strength of a semiconductor device, a field limiting ring (Field Limiting Ring: FLR) is known. Hereinafter, a field limiting ring is simply called “FLR” for simplicity of description. The FLR is a layer provided in a semiconductor substrate to surround an active region in which an element is formed, and has a conductivity type different from an inherent conductivity type of the substrate. The FLR is also called a floating diffusion layer. When using a semiconductor substrate of an N-type, for example, P-type impurities are provided in a ring shape surrounding an active region for doping, so that a P-type FLR is formed. When the FLR is provided, a depletion layer is formed and extends at the p-n junction between the FLR and the semiconductor substrate, and accordingly, dielectric strength is enhanced. Improvement of an FLR is disclosed in Japanese Patent Application Publication No. 2000-114549 (JP 2000-114549 A), for example.